There has been known a method for using an acetonitrile-water mixture as a solvent in production of propylene oxide from hydrogen peroxide and propylene, disclosed in “FY2001 Report of Development of Non-halogen Chemical Process Technology, pages 168 through 210; Development of Next-Generation Chemical Process Technology” and Japanese Unexamined Patent Application Publication No. 285055/2004 (Tokukai 2004-285055; published on Oct. 14, 2004). Further, there has been known a method for using an acetonitrile-water mixture as a solvent, synthesizing hydrogen peroxide from hydrogen and oxygen in a reactor, and reacting the hydrogen peroxide with propylene in the same reactor, disclosed in “FY2002 Report of Development of Non-halogen Chemical Process Technology, pages 161 and 175; Development of Next-Generation Chemical Process Technology” and Japanese Patent Application No. 345450/2006 (Tokugan 2006-345450).